Traditionally, pressed glass items such as television panels, lighting face plates and reflectors, and various consumer items utilize a metal ring, or shell in the case of TV panels and similarly shaped objects, to both strip the glass from the plunger and close off the cavity at the early points of filling the mold with the material to be press-molded. This necessitates relative motion between the plunger and ring as the plunger is pushed through the ring to achieve fill in the cavity.
The interface where the ring and plunger meet, commonly referred to as the bore area, is typically the sight, and source of a host of forming defects in the molded article resulting from: a) the relative motion between the plunger and ring which is necessary for pressing; b) the presence of a non-zero dimensional gap between the plunger and the ring at operating temperatures due to an imperfect ring to plunger fit, commonly referred to as ring fit; and c) non-uniform or non-optimal forming temperatures.
These problems may arise during steady state operation, during start-up when tooling experiences significant thermal growth, or during process transients.
Additionally, a ring fit on a typical consumer article may take 2-6 hours to hand-fit by skilled workers in the plant mold shop. This may then require additional repair on the production line, often referred to as hot repair, once it has been tried on the press in order to fine tune the ring fit so as to minimize defects on the inner edge of the pressed articles. Rarely in the glass industry is the perfect fit achieved or maintained for an extended period of time. This long standing problem of maintaining ring fit is attributable to a multitude of causes including thermal and material homogeneity, irreversible plastic flow in tooling material, article geometry, and glass composition.
Presently, ring materials are chosen based on their dimensional stability and wear characteristics relative to the plunger material. Prior art teaches that the ring be made of softer material than the plunger so that wear takes place largely in the more disposable ring. Thus, inherent ring strength and ring life considerations often become secondary. Typically cast iron, with its favorable wear characteristics and its relatively low cost, is used as a ring material despite the fact that its usefulness for certain applications is reduced by cast iron's low corrosion resistance.
Inner edge related defects on the pressed article, due to ring/plunger interface problems, are probably responsible for an overall 10% loss in acceptable product selections of a given production run. This is especially so in vehicle face plates/reflectors and consumer article production. Poor ring/plunger fit is not the only culprit for loss. The use of certain release and lubricating compounds in the ring/plunger fit area to facilitate removal of the pressed article from the mold, often leads to unacceptable residue on the articles and thereby accounts for further selection loss.
The following references disclose various methods of mold ring alignment.
U.S. Pat. No. 2,966,007 (Sherts et al.) describes an apparatus for pressing glass articles which includes a mold ring, a valve received within the mold ring, a plunger ring and a plunger received within the plunger ring having means for adjustably aligning the pressing plunger and the pressing valve by vertically aligning the inner walls of the two rings. Once vertical alignment of the plunger and valve is obtained the apparatus is locked into place.
U.S. Pat. No. 4,272,273 (Trahan et al.) provides for another adjustable glassware forming machine wherein a floating plunger mechanism is connected to a base plate which is vertically adjustable through the use of a jack screw. The bottom, or base plate, is also floating, in that it is capable of horizontal movement to accommodate slight horizontal movement of the plunger mechanism as is required in order to align the vertically movable plungers with their associated blank molds.
U.S. Pat. No. 4,786,444 (Hwang) describes an apparatus which provides for automatic alignment of mold elements. That is, this apparatus has an alignment means for bringing opposing mold halves together. This apparatus is able to form cylinder-and prism-free lenses.
Lastly, U.S. Pat. No. 4,888,040 (Douglas et al.) describes a glassware forming machine wherein an assembly is shown which provides for continuous engagement between a neck ring and guide ring, in which the guide ring is subject to wear. This continuous engagement is maintained by using spring elements.